This invention relates to adaptive filters and more particularly to apparatus for generating weighting coefficients for sampled data filters such as finite impulse response filters.
Sampled data filters perform weighting and summing operations on successive input samples to produce filtered replicas, y(n), of the input samples, x(n), generally defined by the equation ##EQU1## The factors a.sub.i and b.sub.i are weighting coefficients applied to delayed input and output samples x(n-i) and y(n-i), respectively, where i connotes the number of sample delay periods. The equation defines the transfer function of a recursive filter. If the rightmost term of the equation is eliminated, the equation defines the transfer function of a non-recussive filter.
It will readily be appreciated that the transfer function of a given filter can be altered by changing the value of the weighting coefficients a.sub.i and b.sub.i. If apparatus is provided to automatically change the filter weighting coefficients in response to some stimulus, the filter transfer function is advantageously made adaptive. For example, sets of weighting coefficients may be stored in a read only memory (ROM). The stimulus may be arranged to address the ROM to provide particular sets of the coefficients to the filter weighting elements, which coefficients would correspond to particular values of the stimulus. If the stimulus corresponds to the frequency content of the signal being processed by the filter, the bandwidth of the filter can be varied responsive to the signal frequency content to enhance the average signal-to-noise ratio.
ROM's are a convenient media for storing and accessing sets of weighting coefficients in adaptive filter circuits. However, if the desired range of filter adaptability is relatively wide and is to be incremented with relatively fine resolution, the size of the ROM may become excessive. For a filter having 15 taps, i.e., 15 weighting circuits, with each weighting coefficient consisting of a six bit binary number, one set of coefficients requires 90 bit locations in the ROM. As few as eleven sets of coefficients will require over 1,000 bit locations and will provide relatively course vernier resolution.
It is an object of the present invention to generate sets of weighting coefficients which are substantially continuously variable with relatively simple hardware making possible adaptive filters which are substantially continuously variable over a given range.